1. Field of the Invention
The present invention relates to aqueous, polyvinyl ester-based emulsion adhesives having a practical pot life, to films of these which have increased water resistance, to the preparation of these emulsion adhesives, to their use for bonding porous and semiporous substrates, and to substrates so bonded.
2. Description of Related Art
Aqueous emulsions of polyvinyl esters, in particular of polyvinyl acetate, are used for gluing wood and other porous substrates. The chemistry of these adhesives, which are produced in large industrial volumes, has long been the subject-matter of patent publications and has been described in technical publications, for example, in Wood Adhesives--Chemistry and Technology, Volume 1, chapter 7, Marcel Dekker, New York, 1983.
A subgroup of polyvinyl ester emulsions comprises those having increased water resistance of their films. The sensitivity of polyvinyl ester-based adhesive bonds to water is due predominantly to the presence of hydrophilic stabilizers, in particular, of polyvinyl alcohol, which are usually used in the production of the adhesive emulsion. One way of achieving an improvement in the water resistance is therefore, for example, to reduce the hydrophilicity of the adhesive by crosslinking the polyvinyl alcohol by means of reactive compounds.
Thus, DE-B 22 61 402 states that a polyvinyl ester emulsion to which a crosslinking-inducing resin, for example, a urea-formaldehyde resin, is admixed gives water-resistant adhesive bonds in combination with a free organic or inorganic acid or an acidic metal salt.
Another method of reducing the hydrophilicity of the adhesive, which has been known for some time, is to copolymerize vinyl esters in the presence of monomers which can be crosslinked with polyvinyl alcohol, such as N-methylol(meth)acrylamide. A method of this type is described, for example, in U.S. Pat. No. 3,301,809. Combination with acidic curing agents, for example, organic acids or acidic ammonium salts, gives water-resistant adhesives.
As curing agents for crosslinkable, polyvinyl alcohol-stabilized polyvinyl acetate adhesive emulsions, U.S. Pat. No. 3,563,851 proposes, in particular, the acidic salts of Al(III) and of Cr(III).
The specifications of DE-C 26 20 738 and DE-A 39 42 628 likewise relate to adhesive systems based on copolymers of vinyl esters with crosslinkable comonomers including N-methylolacrylamide, which crosslink with acidic catalysis. These adhesives likewise require the use of strong water-soluble acids or metal salts thereof, in particular salts of Cr(III), Al(III), Fe(III) and Zr(IV). According to DE-A 39 42 628, the achievement of adhesive bonds which are resistant to hot water, in accordance with the BS EN 204 test standard, strength group D4 (formerly DIN 68602 B4), additionally requires the copolymerization of highly branched vinyl carboxylates if acidic aluminum salts are used as curing agents.
In contrast to these adhesive systems, the crosslinking of which is catalyzed exclusively by acidic curing agents, it also is known subsequently to add to the emulsions various additives which increase the water resistance of adhesive bonds produced using these emulsions. The effect of these additives is due to crosslinking reactions involving the polyvinyl alcohol.
One group of these additives includes hydrophobic, aliphatic, or aromatic polyisocyanates. U.S. Pat. No. 3,931,088 mentions the addition of polyfunctional isocyanate compounds, for example, toluidene diisocyanate, to aqueous adhesive emulsions containing polyvinyl alcohol. These compounds are added in solvents.
As a further development, EP-A 0 206 059 describes a water-dispersible polyisocyanate preparation in which some of the isocyanate groups are masked by emulsifiers. The addition of this product to polyvinyl acetate emulsions gives adhesive bonds which are resistant to hot water and are classified in BS EN 204 strength group D4.
A further group of additives which improve the water resistance of emulsion adhesives comprises polyfunctional aldehydes.
In U.S. Pat. No. 3,932,335, polyvinyl ester emulsions which give water-resistant adhesive bonds are obtained by adding glyoxal in combination with water-soluble polyols.
EP-A 0 191 460 describes polyvinyl alcohol-stabilized polyvinyl acetate emulsions which are crosslinked with glutaraldehyde and give films which are resistant to boiling water for at least 30 minutes.
The above-mentioned curing agent systems have some disadvantages for the user. The use of acidic curing agents results in a drop in viscosity of the emulsions, in particular, when relatively large amounts are used. In addition, relatively large amounts of these additives, particularly in the case of chromium and aluminum salts, cause an undesired increase in the powder point of the emulsions and a drop in the short-term bonding strength. Any acidic salt solution admixed separately can cause corrosion in the application equipment. In addition, the aqueous solutions of the salts represent hazardous materials during application due to their irritant action. The blue-green adhesive joint formed on use of chromium(III) salts, which are furthermore toxic, adversely affects the visual impression of glued articles, in particular in the case of pale wood types.
The use of urea-formaldehyde resins as described in DE-B 22 61 402 results in gradual liberation of formaldehyde from the adhesive. Use of the crosslinking polyisocyanates as described in EP-A 0 206 059 results in gradual, undesired foaming of the preparations during application which is caused by released carbon dioxide, as described herein in comparative examples V1, V2 and V3, Table 1.
Some of the emulsion adhesives prepared as described in EP-A 0 191 460 with addition of polyaldehydes satisfy the existing demands. But the systems that have adhesive bonds with high water resistance cannot simultaneously achieve the requisite practical pot lives of at least 8 hours. In addition, systems of this type produce a strong odor nuisance due to the high content of free polyaldehyde, as shown in comparative example V4, Table 1.